Inhibition of popcorn polymerization of vinylpyridines



INHIBITION OF POPCORN POLYMERIZATION OF VINYLPYRIDINES Clyde W. Mertz, Borger, Tex., assignor to Phillips Petroleum Company, a corporation of Delaware N Drawing. ApplicafionMay 22, 1953 Serial No. 356,869

17 Claims. (Cl. 260-290) This invention relates to a process for inhibiting proliferous type polymerization of polymerizable heterocyclic compounds having a hetero nitrogen atom. In one of its aspects, this invention relates to the inhibiting of proliferous polymerization of vinyl-substituted heterocyclic nitrogen compounds. In a further aspect this invention relates to the inhibiting of the formation of proliferous type polymers in vinylpyridine compounds.

The words inhibit and stabilize as used in this specification and the claims appended thereto are used to means that the formation of popcorn polymer is materially reduced or prevented and do not imply any mechanism for accomplishing this purpose.

p In the processing of Z-methyl-S-vinylpyridine, vinylpyridine, vinyl-substituted heterocyclic nitrogen com-" pounds, and other alkene, alkadiene and alkyne substituted heterocyclic nitrogen compounds by various means, including distillation, polymerization occurs which manifests itself in two ways. Soluble polymers are formed which are evidenced by an increase in viscosity, and ultimately, by forming a solid mass. Also formed are insoluble polymers or proliferous polymers referred to in the trade and herein as popcorn polymers. The popcorn polymer is insoluble in the heterocyclic nitrogen compound and in other organic compounds, and when in contact with a monomer, it continues to grow. This problem is particularly acute in the manufacture and processing of vinyl-substituted pyridines obtained by the dehydrogenation of the corresponding ethyl substituted pyridines. Popcorn polymer accumulates or grows very rapidly in distillation processes, resulting in the loss of valuable material and causing pipes, condensers, and equipment to be clogged.

When Z-methyl-S-vinylpyridine (MVP) is made from 2-methyl-5-ethylpyridine (MEP) by dehydrogenation, the resulting mixture contains, besides the major constituents MVP and MEP, small amounts of picolines, divinylpyridines, and some unidentified materials probably other'diolefin substituted pyridines which initiate the proliferou's type polymerization known to the art as popcorn polymer.

In accordance with this invention, for the purpose of overcoming difficulties due to proliferous polymer formation tendencies of these heterocyclic nitrogen compounds, it is proposed to add a novel inhibiting agent. I have discovered that readily polymerizable heterocyclic nitrogen compounds can be effectively inhibited from the formation of popcorn type polymerization during fractionation and storage by adding an organic nitro compound selected from the group of compounds represented 1 NO --RNHR' wherein R represents phenylene or an alkyl substituted phenylene and R represents alkyl, phenyl, alkyl substituted phenyl or hydrogen and said compound will for the purpose of this specification and the claims be designated as a nitrophenylamine. Among the nitrogen organic compounds which are useful in the practice of my Fair-exited Dec. 30, 1958 invention are ortho, r'neta and para-nitroaniline, ortho, meta and para-nitrodiethylaniline, ortho, meta and paranitrodimethylaniline, ortho, meta and para-nitrodiphenylamine, nitro-4-methyldiphenylamine, ortho, meta and para-nitrotoludiene, ortho, meta and para-nitroxylidiene, 3 nitro-o-toludiene, 4 nitro-o-toludiene, 5 nitro-o-toludiene, 6-nitro-o-toludiene, 2-nitro-r'n-toludiene, 6-nitro-rn-v toludiene, Z-nitro-p-toludiene, 3-nitro-p-toludiene, Z-nitrodiphenylamine, 3-nitro-diphenylamine, 4-nitro-diphenylamine, 3-nitro-3-methyl-diphenylamine, 4-nitro-4-methyldiphenylamine, 2-nitro-3-methyl-diphenylamine, 3-nitro- 4-methyl-diphenylamine, 4-nitro-2-methyl-diphenylamine and like compounds.

The polymerization which is inhibited by the novel stabilizing agents disclosed herein is primarily that which forms an insoluble, hard, porous and opaque material often referred to as popcorn polymer. However, polymers are also formed which are soluble in the heterocyclic nitrogen reactants and which become apparent by an increase in viscosity. As is seen from the examples, soluble polymer formation is also inhibited by the disclosed nitrophenylamine compounds.

One group of polymerizable heterocyclic nitrogen compounds which can be inhibited in accordance with my invention is the monoand divinylpyridines, with the vinyl group being present in any of the several positions in the pyridine ring. Alkyl groups can be present on the ring, or on the alpha carbon atom of the vinyl group, but the number of carbon atoms in the combined alkyl groups should generally not be greater than 12. These alkyl groups are preferably methyl and ethyl groups. Sub stituents attached to carbon atoms in the ring can be selected from the group consisting of hydrogen, alkyl, vinyl, and isopropenyl (alpha-methylvinyl) groups; at least one and not more than two of said groups being vinyl or alpha-methylvinyl; and the total number of carbon atoms in the alkyl groups being not greater than 12. Examples of such compounds are 2-vinylpyridine; 3-vinylpyridine, 4-vinylpyridine, trimethyl-S-vinylpyridine; 3,4,5,6 tetramethyl-Z-vinylpyridine; 3-ethyl-5-vinylpyridine; 2,6-diethyl-4-vinylpyridine; 2-isopropyl-4-nonyl-5- vinylpyridine; Z-methyl-S-undecyl-3-vinylpyridine; 3-dodecyl 4,5 divinylpyridine; 2,4-dimethyl-5,6-diphenyl-3- vinylpyridine; 2-decyl-5-(alpha-methylvinyl) pyridine; 3,5-di (alpha-methylvinyl) pyridine; similar mono and di substituted alkene, alkadiene, and alkyne pyridines.

Other polymerizable heterocyclic compounds containing a hetero nitrogen atom included within the scope of this invention are those vinyl and alpha-methyl-vinyl substituted heterocyclic nitrogen compounds in which the ring structure is unsaturated, partially saturated, and completely saturated. Examples include alkene, alkadiene,

and alkyne substituted quinolines, iso-quinolines, piperi-= the alkene, alkadieneor alkyne substituents will be at-v tached to a ring carbon atom. However, in compounds wherein the ring nitrogen atom is a secondary nitrogen ing a substantial proportion 'of a vinylpyridine compound,-

Examples of vinylpyridine compounds contained in such mixtures are 2-vinyl 4,o-dimethylpyridine, 2-methyl-4- vlnylpyridine, 2-methyl-5-vinylpyridine, 5-ethyl-2-vinylpyridine, 5-amyl-2-vinylpyridine. The heterocyclic compound containinga hetero nitrogen atom of particular commercial importance at the present time is Z-methyl- S-vinylpyridine (MVP).

In carrying out the invention, solution of the inhibiting agent in the heterocyclic nitrogen compound can be accomplished in any suitable manner. The concentration of the nitrophenylamine to be used will depend upon numerous factors such as temperature, time, and economic considerations. In practice a range of concentrations of from 0.01 percent to 5.0 percent by weight of the nitrophenylamine will be employed, all percentages being based on the weight of the heterocyclic nitrogen compound being inhibited. Also, the amount will depend on the particular heterocyclic nitrogen compound concerned, and/or as will be apparent from the following examples, upon the particular nitrophenylamine being utilized.

Example I A mixture comprising approximately 95 weight percent of 2-methyl-5 vinylpyridine, and the balance being comprised of 2-methyl-5-ethylpyridine, divinylpyridine, dimethylpyridine, picolines, and the like as obtained from a dehydrogenation plant was vacuum flashed at less than 50 mm. mercury pressure and at 200 F. in a laboratory distillation column. The bottoms product containing a large percentage of divinylpyridine was recovered for use as a polymerinitiator.

Approximately one percent, based on the weight of the monomer mixture being initiated, of the bottoms product from the laboratory still was added to a portion of the 95 percent MVP material along with a small seed of popcorn polymer. The seeded mixture was stored at 190 F. for a period of 12 days with the results as shown in the table below.

Unless the bottoms product is used as an initiator, the behavior of the seeded product was unpredictable, it taking 24 to48 hours or longer for the complete formation of popcorn polymer. In every case where the initiator material wasadded, the seeded monomer was completely converted to popcorn polymer in less than hours.

Material Observation (1) Control comprised of 95% MVP material plus 1% still bottoms and a popcorn seed.

All popcorn in less than 10 hours.

an (5) (1) plus 0.2 wt. percent 2 nitrodlphenylamine.

Very viscous; no popcorn polymer infill days. 0.

Slightly viscous, no polymer in 10 days. Slightly viscous in 8 days, no

popcorn polymer in 11 days.

popcorn From the foregoing, it is apparent that nitrophenylamines are effective inhibitors of popcorn, or proliferous polymer, formation when heterocyclic compounds having a hetero nitrogen atom are maintained at elevated temperature, and most remarkably, even when the test solutions are seeded to make the test conditions more severe.

This invention accordingly concerns a method for treating polymerizable heterocyclic nitrogen compounds with nitrophcnylamineup to 400 F. as well as at room temperatures 'orbelow. ln accordance with this inven' tion organic mixtures containing polymerizable heterocyclic nitrogen compounds mixed with other heterocyclic nitrogen compounds, or purified polymerizable heterocyclic nitrogen compounds, can be stored over long periodsof time preferablyatroom temperature. Also, when. it is desired to separate a particularheterocyclic nitrogen example, in the following manner. In the preparation of 2-methyl-5-vinylpyridine from 2-methyl-5-ethylpyridine by dehydrogenation, the principal separation is a combined steam and vacuum distillation to make a separation between the MVP and the MEP. Because this is a difficult separation, due to the proximity of the boiling points of the stated components, considerable polymerization of the MVP occurs, and consequently, after a short period of time, operation of the column becomes impossible due to insoluble polymer formation in the lower part of the fractionation column and in the kettle. However, by the use of a nitrophenylamine in accordance with this invention, shut-downs due to the formation of insoluble polymers can be largely eliminated. The nitrophenylamine can be added directly to the feed of the distillation column or it may be added to the reflux of said column or a combination of the two. The amount of the nitrophenylamine to be used may vary from 0.01 to5.-0 percent and generally from 0.05 to 5.0 percent by weight based on the weight of the heterocyclic nitrogen compound. I prefer to use between 0.5 and 1.0 percent inhibitor.

It will be understood that the foregoing disclosure is byway of illustration only and that other embodiments within the scope of the invention will occur to those skilled in the art. Those in the art will appreciate, for instance, that for the purpose of this invention, alkene, alkadiene, and alkyne substituted heterocyclic compounds having a hetero nitrogen atom further substituted with non-interfering groups, for example, halo-, nitro-, amino-, hydroxyl-, and carboxyl-, are the same as alkene, alkadione, and alkyne heterocyclic nitrogen compounds not further substituted.

I claim:

1. A method which comprises stabilizing againstproliferous polymerization during fractionation and storage a compound selected from the group consisting of monovinylpyridine and alkyl substituted monovinylpyridines wherein the total carbon atoms in alkyl substituents does not exceed 12, by treating said compound with inhibiting amounts of an organic compound of the general formula wherein R is selected from the group consisting of phenylene and alkyl substituted phenylene and wherein R is selected from the group consistingof alkyl, phenyl, alkyl substituted phenyl and hydrogen.

2. A method which comprlses dissolving inhibiting amounts of a nitrophenylamine of the general formula wherein R is selected from the group consisting of phenylene and alkyl substituted phenylene and wherein R isselected from the group consisting of alkyl, phenyl, alkyl substituted phenyl and hydrogen into a proliferous polymerizable compound selected from the group of compounds consisting of monovinylpyridine and alkyl substitutdmonovinylpyridines wherein the total carbon atomsin alkyl substituents does not exceed 12, and thereaft'erfractionating the solution to recover the polymerizable compound in a readily polymerizable form.

3. The method of claim 2 wherein the nitrophenylamine is dissolved in an amount to provide a solution percent by weight, based on the compound of the above general formula, of a nitrophenylamine of the formula NO R--NHR where R is selected from the group consisting of phenylene and alkyl substituted phenylene and R is selected from the group consisting of alkyl, phenyl, alkyl substituted phenyl and hydrogen and effecting said distillation in the presence of said nitrophenylamine.

5. The process of claim 4 wherein the compound being separated is 2-rnethyl-5-vinylpyridine.

6. A process for inhibiting the formation of proliferous polymers in a mixture of organic compounds, said organic compounds being comprised of 2-methyl-5-vinylpyridine, divinylpyridine, and Z-methyl-S-ethylpyridine, which said process comprises adding to said mixture of organic compounds inhibiting amounts of a nitrophenylamine compound of the formula NO -RNHR' wherein R is selected from the group consisting of phenylene and alkyl substituted phenylene and wherein R is selected from the group consisting of alkyl, phenyl, alkyl substituted phenyl and hydrogen.

7. The process of claim 6 wherein the nitrophenylamine compound is nitroaniline.

8. A polymerizable heterocyclic nitrogen base selected from the group consisting of compounds consisting of monovinylpyridine and alkyl substituted monovinylpyridines wherein the total carbon atoms in alkyl substituents does not exceed 12, said heterocyclic nitrogen base being stabilized by the addition of a stabilizing amount of a nitrophenylamine having the formula where R is selected from the group consisting of phenylene and alkyl substituted phenylene and R is selected 6 from the group consisting of alkyl, phenyl, alkyl substituted phenyl and hydrogen.

9. The composition of claim 8 wherein the amount of nitrophenylamine is in the range of from 0.05 and 5.0 weight percent based on the weight of the nitrogen base.

10. A composition of matter comprising a mixture of organic compounds, said mixture comprising Z-methyl-S- vinylpyridine, 5-ethyl-2-methylpyridine, and divinylpyridine, said mixture having incorporated therein 0.05 to 5.0 percent by weight based on the weight of the Z-methyl- 5-vinylpyridine, a nitrophenylamine of the formula wherein R is selected from the group consisting of phenylene and alkyl substituted phenylene and R is selected from the group consisting of alkyl, phenyl, alkyl substituted phenyl and hydrogen.

11. The composition of claim 10 wherein the nitrophenylamine is nitroaniline.

12. The process of claim 6 wherein the nitrophenylamine is para-nitroaniline.

13. The process of claim 6 wherein the nitrophenylamine is nitrodiphenylamine.

14. The process of claim 6 wherein the nitrophenylamine is Z-nitrodiphenylamine.

15. The composition of claim 10 wherein the nitrophenylamine is para-nitroaniline.

16. The composition of claim 10 wherein the nitrophenylamine is nitrodiphenylamine.

17. The composition of claim 10 wherein the nitrophenylamine is Z-nitrodiphenylamine.

References Cited in the file of this patent UNITED STATES PATENTS Mahan June 27, 1950 Kauffman June 12, 1951 OTHER REFERENCES UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,866,789 December 30, 1958 Clyde W, Mertz It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column. 6, list of references cited, under he heading UNITED STATES PATENTS, add the following reference:

2,592,625 wagner=== April 15,, 1952 Signed and sealed this 6th day of October 1959,

SEAL) Attest:

KARL Ha AXLINE ROBERT C. WATSON Attesting Ofi'lcer Commissioner of Patents 

1. A METHOD WHICH COMPRISES STABILIZING AGAINST PROLIFEROUS POLYMERIZATION DURING FRACTIONATION AND STORAGE A COMPOUND SELECTED FROM THE GROUP CONSISTING OF MONOVINYLPYRIDINE AND ALKYL SUBSTITUTED MONOVINYLPYRIDINES WHEREIN THE TOTAL CARBON ATOMS IN ALKYL SUBSTITUENTS DOES NOT EXCEED 12, BY TREATING SAID COMPOUND WITH INHIBITING AMOUNTS OF AN ORGANIC COMPOUND OF THE GENERAL FORMULA 